mxmirror: Avoid perl warning

When the source jbod is not in mxmounts, we currenty get

    **ERROR: unknown jbod: X2083
    Use of uninitialized value in concatenation (.) or string at /usr/sbin/mxmirror line 410, <F> line 240.
    Use of uninitialized value in concatenation (.) or string at /usr/sbin/mxmirror line 410, <F> line 240.
    **ERROR: CAN'T PARSE FROM/TO: X2083 X2076

because the `undef` result of `convert_to_sourcepath` is
used by `convert_to_destinationpath`.

Bark out early when the source path is not parsable.

This changes output to

    buczek@pummelfee:~/git/mxtools (master)$ mxmirror/mxmirror
    **ERROR: unknown jbod: X2083
    **ERROR: CAN'T PARSE FROM: X2083 X2076

mxmirror/mxmirror

@@ -487,10 +487,13 @@ sub read_mirrormap {
         if(($from, $to, $args) = /^(\S+) (\S+)\s*(.*)$/) {
 
             $source      = convert_to_sourcepath($from);
+            unless ($source) {
+               print STDERR "**ERROR: CAN'T PARSE FROM: $_\n";
+               next;
+            }
             $destination = convert_to_destinationpath($to, $source);
-
-            unless($source and $destination) {
-               print STDERR "**ERROR: CAN'T PARSE FROM/TO: $_\n";
+            unless($destination) {
+               print STDERR "**ERROR: CAN'T PARSE TO: $_\n";
                next;
             }